CS-5891: ICS file not attached with the invitation mail

Problem analysis:
* When trying to send an invitation to participants just when saving the event, ical4j can not build the iCalendar model.
* Since the method exportEventCalendar in ICalendarImportExport class search for already stored events, it could not retrieve the current event which is not yet stored.

Fix description:
* Send the mail after saving the event using a new method sendInvitationMail.
* Existant SendInvitation method will just update some properties.